1. Field of the Invention
The present invention relates generally to direct sequence spread spectrum (code division multiple access) receivers. This invention is particularly concerned with a direct sequence spread spectrum technique for cellular mobile communications systems in which signals are severely affected by Rayleigh fading.
2. Description of the Related Art
Commercial interest in direct sequence spread spectrum communication systems has recently risen due to their potential ability to provide service to more users than is offered by other multiple access techniques. In the cell-site station of the DS/CDMA system, a data symbol is spread by multiplying it with higher frequency orthogonal pseudo-random number (PN) sequences assigned to the cell site as well as with orthogonal Walsh codes assigned to the channel over which the spread signal is transmitted. In order to enable the mobile station to implement synchronous acquisition and tracking operations, a pilot signal is superimposed on the data symbol sequence. At the mobile station, a sliding correlation technique is used to shift the phase timing of a local PN sequence by a predetermined amount each time a correlation is taken between the received and local sequences and determine the correct phase timing for the local sequence when the correlation exceeds some critical value. Once synchronization is established, the phase difference is monitored and maintained to within a fraction of the chip interval. During transmission, the signal undergoes reflections and scattering from various land structures, producing a complex pattern of standing waves due to mutual interference of the multipath signals at the mobile station. As a result, the propagation path of the signal exhibits a field intensity distribution which is approximated by the Rayleigh distribution. Thus, the signal experiences a phenomenon called "Rayleigh fading" and the envelope of the signal at the mobile station as well as its phase violently fluctuate.
Under such unfavorable conditions, the transmitted signal is subjected to corruption by noise and the chip-rate phase timing of the signal deviates from instant to instant from the local timing of the receiver.